1. Field of the Invention
This invention relates to combustion of gaseous and vaporous fuels, particularly fossil-derived fuels, to reduce NO.sub.x emissions. This invention involves oscillation of fuel or combustion air provided to a burner to generate successive fuel-rich and fuel-lean zones in a flame.
2. Description of the Prior Art
Nitrogen oxides, primarily NO and NO.sub.2 (NO.sub.x) are a major cause of air pollution as a result of combustion processes.
It is generally recognized that fuel-rich and highly fuel-lean combustion conditions reduce NO.sub.x formation. There have been several attempts to utilize these conditions in combustion processes, usually involving multi-stage combustion. Multi-stage combustion wherein additional air or fuel is added downstream from the primary combustion is exemplified by the following publications. U.S. Pat. No. 4,403,941 teaches reduction of NO.sub.x in a multi-stage combustor having in succession a primary burner with fuel-rich incomplete combustion, a secondary burner with a fuel richer reducing combustion zone, and an after burner providing excess air for completion of combustion. U.S. Pat. No. 4,500,281 teaches another multi-stage combustion process wherein NO.sub.x emissions are reduced by having serially in open communication a fuel-rich combustion zone, introduction of a combustion catalyst, a second fuel-rich combustion zone, and a fuel-lean combustion zone. European Patent Publication 73265 teaches NO.sub.x reduction by having a fuel-rich primary combustion zone which abruptly terminates with introduction of combustion products and additional secondary air to a fuel-lean secondary combustion zone with prevention of backflow into the primary combustion zone.
U.S. Pat. No. 3,982,393 teaches cyclic combustion, as in an internal combustion engine, wherein some of the cylinders are provided fuel-rich mixtures and the remainder of the cylinders are provided fuel-lean mixtures with discharge from only the fuel-rich fed cylinders passing to an afterburner, the number of cylinders under each regime changing with low load engine operation and medium and high load engine operation. U.S. Pat. No. 3,708,980 teaches an internal combustion engine wherein alternate cylinders are supplied rich and lean fuel mixtures with the exhaust gases from the fuel-rich fed cylinders passing through an exhaust gas treating unit for reduction of NO.sub.x and the exhaust gas from all cylinders then conducted through an exhaust gas treating unit oxidizing hydrocarbons and carbon monoxide.
U.S. Pat. No. 4,408,982 teaches reduction of NO.sub.x produced by furnace combustion with alternate high/low firing rates of fuel and oxidant involving aspiration of furnace gases into the oxidant jet with delay of oxygen injection at the start of the high firing rate. This patent teaches that alternate high-low firing rates of fuel and oxidant increases NO.sub.x emissions with oxygen enrichment. U.S. Pat. No. 4,637,792 is exemplary of pulse combustion patents wherein the rate of introduction of combustion mixtures to a combustion chamber is modified, but like the teachings of the '982 patent, the proportion of fuel and oxidant in the introduced mixtures remain constant.